U.S. Pat. No. 5,789,646 discloses a process for the double bond isomerization of a vinylidene olefin, comprising contacting a feed comprising the vinylidene olefin with an isomerization catalyst which comprises a molecular sieve which is a zeolite in the hydrogen form. The zeolite specifically disclosed is an H-ZSM-5 having a silicon/aluminum atomic ratio of 25. The pore size of H-ZSM-5 is that of the two channels which interconnect, viz. channels of 0.53×0.56 nm diameter and channels of 0.51×0.51 nm diameter (cf. W M Meier and D H Olson, “Atlas of Zeolite Structure Types”, 2nd Revised edition (1987), published by the Structure Commission of the International Zeolite Association, p. 100–101). U.S. Pat. No. 5,789,646 considers the influence of the acidity of a solid acid on its performance as a catalyst in the vinylidene isomerization and suggests that accordingly the catalysts may be positioned on an acidity scale. It is striking that in the working examples of U.S. Pat. No. 5,789,646 the results reported for the H-ZSM-5 catalyst and for a crosslinked sulfonic acid catalyst are “essentially identical”, which would suggest that widely different solid acid catalysts are essentially equally positioned on the acidity scale.
U.S. Pat. No. 4,697,040 discloses a process for the double bond isomerization of a vinylidene olefin, comprising contacting a feed comprising the vinylidene olefin with an isomerization catalyst which comprises a molecular sieve which is a zeolite in the sodium form. The zeolite of U.S. Pat. No. 4,697,040 is a specific Y type zeolite, designated LZ-Y52 (trademark), which has a pore or channel size of 0.74 nm (cf. W M Meier and D H Olson, “Atlas of Zeolite Structure Types”, 2nd Revised edition (1987), published by the Structure Commission of the International Zeolite Association, p. 62–63).